Thymus-derived (T) lymphocytes with a broad cytolytic capability, including the capacity to kill unmodified syngeneic target cells (both normal and neoplastic), are generated when "purified" T cells from specifically immunized rats are stimulated in vitro by soluble antigens of the intracellular bacterial parasite, Listeria monocytogenes. Stimulation is dependent upon the adsorption of the antigens to accessory cells that share certain MHC-coded products with the responder lymphocytes. The cytolytic effector cells have the cell membrane receptor profile, FCR minus, Ig minus, W3/13 plus, W3/25 minus, MRC Ox-8 plus, MRC Ox-3 plus. Questions arise whether T cells with the same lytic potential are generated in vivo, and if so, whether they have a biologically significant role in the mediation of or control of the delayed inflammatory response and cellular resistance to infection. Preliminary evidence suggests an affirmative answer to the first question. Thus, nonphagocytic, nylon-wool-nonadherent Fc receptor negative cells harvested from the peritoneal cavity of Listeria immune rats can kill unmodified target cells without further stimulation in vitro. The plan is to characterize and compare these cytolytic effector cells with those generated in vitro, and to ascertain their phlogistic properties, protective capacity and suppressor activities in Listeria infected rats.